We report the clinical presentation and the morphologic, histochemical, and immunophenotypic characteristics of seven patients with acute leukemia who had trisomy/tetrasomy 13 as the sole cytogenetic abnormality in their leukemia. Five patients had trisomy 13 at diagnosis of acute leukemia. All five of these patients had undifferentiated leukemias. The sixth patient, who had French-American-British (FAB) type M2 acute nonlymphocytic leukemia (ANLL), and the seventh patient with biphenotypic acute leukemia developed the trisomic clone as a new abnormality late in the course of their disease. A review of the literature revealed 28 previously reported hematologic malignancies with trisomy 13 or tetrasomy 13q as a solitary cytogenetic abnormality. Trisomy 13 appears to represent another rare but nonrandom cytogenetic abnormality in acute leukemia. In our series trisomy 13 is largely associated with acute leukemia with little myeloid or lymphoid differentiation.